The present invention relates to the art of taking core samples from within a well bore and, more particularly, to apparatus for taking a plurality of core samples from the sidewall of a borehole. Still more particularly, the present invention relates to core sampling apparatus capable of taking samples from within a deviated borehole.
The use of a sidewall core gun to take a formation sample from the sidewall of a borehole is well known. U.S. Pat. Nos. 2,928,658; 2,937,005; 2,976,940; 3,003,569; 3,043,379; 3,080,005; and 4,280,568 disclose various types and aspects of sidewall core guns. Typically, a sidewall core gun comprises a cylindrical gun housing suspended on a wireline at a predetermined depth within the borehole. Included within the housing oriented radially outwardly is a plurality of means for taking core samples. In U.S. Pats. Nos. 2,937,005; 3,003,569; and 4,280,568, the sampling means comprises a hollow coring bullet which, when launched into the sidewall, will cut away a sample of the sidewall and retain that sample within the hollow center of the bullet. The bullet remains attached to the gun housing via cables whereby retrieval of the gun pulls the imbedded coring bullet from the sidewall.
In U.S. Pats. Nos. 3,043,379 and 3,080,005, the sampling means comprises a plurality of shaped charges arranged within the gun housing so as to cut formation samples from the sidewall. The shaped charges are oriented in a converging pattern whereby a high velocity stream of hot gases and particles emitted by the burning shaped charges severs a formation sample from the sidewall. The sample is collected in a pocket within the gun housing immediately below the shaped charges.
Still another sampling means, such as is shown in U.S. Pats. Nos. 2,928,658 and 2,976,940, comprises a combination of coring bullets and shaped charges. In this case, coring bullets are used to secure a sample from the sidewall and shaped charges are used to dislodge the sample or the coring bullet from the sidewall.
Conventionally, as in the case of the sidewall core guns described in the above-noted patents, the core gun is suspended within the borehole on a wireline. Accordingly, the coring bullets and shaped charges are fired by electrical firing means. Such firing means comprises, for example, electrical wires which terminate in a resistance-heated wire, such as a nichrome wire. The resistance-heated wire, when energized, ignites a detonator, which, in the case of the coring bullet, detonates an explosive charge. Typically, the explosive charge is a gas generator which launches the coring bullet by means of gaseous expansion acting on a rearward surface thereof. In the case of a shaped charge, the detonator fires the shaped charge or an ignition train leading thereto.
Core guns which employ hollow coring bullets without the aid of shaped charges are particularly susceptible to problems associated with retrieval of the coring bullets from the surrounding formation. The use of coring bullets without shaped charges is desirable because of the additional expense and complexity added to the gun by the shaped charges. However, if the explosive charge used to launch the coring bullet is excessively powerful, the coring bullet may lodge deep within a consolidated formation. When the gun is retrieved, the cables connecting the coring bullets to the gun housing, being under substantial tension, may sever, leaving the bullets buried within the formation. In other cases, where formation resistance to the intruding coring bullet is minimal, the connecting cable may be severed by the force of the explosive charge alone. Thus, proper control of the force generated by the explosive charge is critical to successful retrieval of the coring bullets.
If the cables connecting the coring bullets to the gun housing are strengthened to facilitate bullet retrieval in consolidated formations, the substantial tension acts first on the wireline connecting the gun housing to the surface, subjecting the wireline to risk of severance. Some hydrocarbon formations are so highly consolidated as to preclude the firing and retrieval of more than one coring bullet at a time without risking severance of the wireline. Thus, taking core samples in a highly consolidated formation can be a time-consuming process.
Each of the sidewall core guns described above is also susceptible to a problem common to all wireline operations--the guns are operable only in vertical or substantially vertical boreholes. The more a borehole is deviated, the less likely it is that one will be able to obtain from it formation samples by use of a sidewall core gun. Hence, it appears that the known prior art methods and apparatus relating to sidewall core guns do not provide adequately for the taking of formation samples from within a deviated borehole.